The ultra-sound sensor measures physical parameters such as location, speed and distance by emitting and receiving an ultra-sound wave. In an ordinary electronic device using an ultra-sound sensor, a micro-processor generates a driving voltage/current signal according to the frequency and magnitude of the to-be-emitted ultra-sound wave for driving the ultra-sound emitter to emit an ultra-sound wave. Then, the ultra-sound receiver receives the reflected ultra-sound wave and the micro-processor further converts the received ultra-sound wave into a voltage/current signal. Normally, the ultra-sound sensor has both functions of receiving and emitting ultra-sound waves.
In some applications, a single ultra-sound sensor is used for both emitting and receiving ultra-sound waves; and in other applications, two or more ultra-sound sensors are employed and signals are received and transmitted between these ultra-sound sensors. No matter application of one single or several ultra-sound sensors, due to the properties of the ultra-sound sensor, the detectable range of the ultra-sound sensor is restricted, especially in close distance detection.
When two or more ultra-sound sensors are employed, one ultra-sound sensor is used for emitting ultra-sound waves, and the other ultra-sound sensor is used for receiving reflected ultra-sound waves. In detection of an object in close distance, the influence of multiple reflections effect of reflective ultra-sound wave may become serious and the ultra-sound receiver has difficulty in correctly distinguishing main reflective ultra-sound wave from multi-reflective ultra-sound wave. So, it is difficulty to perform appropriate control.
When one single ultra-sound sensor is employed, a reverberation time or a ringing time will affect the shortest detectable distance. As disclosed above, the micro-processor will output a driving signal to the ultra-sound emitter. After the driving signal stops, the ultra-sound emitter is still in ultra-sound vibration for a short interval (i.e. the reverberation time or the ringing time) due to the inertial oscillation. That is, after the driving signal stops, the ultra-sound emitter still emits ultra-sound waves during the reverberation time or the ringing time and such occurrence is referred as “reverberation” or “ringing”.
Let the applications using one single ultra-sound sensor be taken for example. The ultra-sound sensor emits and receives ultra-sound waves. If a reflective ultra-sound wave is received within the reverberation time, the micro-processor will not be able to distinguish the reflective ultra-sound wave due to the overlap between the reflective ultra-sound wave and the emitted ultra-sound wave within the reverberation time. Therefore, in the application using one single ultra-sound sensor, it is to avoid detection of an object within a distance related to the reverberation time (i.e. the distance is equal to a multiplication of an ultra-sound speed and the reverberation time). In other words, the reverberation time will restrict the shortest detectable distance of the ultra-sound sensor.